References may be made to previous works on different catalytic materials such as Fe-MFI [J Péréz-Ramiréz et al. Catal. Commun. 3(2002) 19-23, J Péréz-Ramiréz et al. J. Catal. 214 (2003) 33-45; J Péréz-Ramiréz et al. Appl. Catal. B: Environmental 35, (2002), 227-234]; LDHs [Dandl & Emig Appl. Catal. A: General 168 (1998) 261-268; Kannan & Swamy Appl. Catal. B: Environmental 3, (1994), 109-116; Armor et al. Appl. Catal. B: Environmental 7, (1996), 39-406; Drago et al. Appl Catal B: Environmental 13 (1997) 69-79; M. C. Roman-Martinez et al. Appl. Catal. A: General 225 (2002) 87-100; J Péréz-Ramiréz et al. Appl. Catal. B: Environmental 25 (2000) 191-203; Catal. Lett. 60, 1999 133-138, K. Karásková et al.Chem. Eng. J 160 (2010) 480-487, S. Kannan, Catal. Surv. Asia 10 (2006) 117-136, F. Kovanda et al. J. Solid State Chem. 179 (2006) 812-823]; SBA-15 based catalysts [Xiaoding Xua et al. Appl Catal B: Environmental 53, (2004), 265-274); Catal. Lett. 93 (2004) 113-120]; Co or Ni supported on MgO [Izumi et al. Chem. Lett. (1998), 727-728, Chem. Commun. (2000), 1053-1054]; Pd-ZSM-5, Fe-ZSM-5 [Izumi et al. Bull. Chem. Soc. Jpn. 74 (2001) 1499-1505, Pirngruber et al. J Catal. 22 (2004) 429-440, Perez-Ramirez et al. Kinet. Catal. 44 (2003) 639-647, J Péréz-Ramiréz et al. Catal. Today 76 (2002) 55-74, I. Melián-Cabrera et al. J. Catal. 238 (2006) 250-259], mixed oxide catalysts [K. Galejová et al. Chem. Pap. 63 (2009) 172-179] iron incorporated aluminophosphate molecular sieves Fe-FAPO-5 (FAPO) [W. Wei et al. J. Catal. 262 (2009) 1-8, W. Wei et al. Microporous Mesoporous Mater. 112 (2008) 193-201], zeloite coating on solis supports [J. C. Jansen et al. Microporous Mesoporous Mater. 21 (1998) 213-226] Rh supported on metal oxides like ZnO or CeO2 [Oi et al. Appl Catal B: Environmental 12 (1997) 277-286, Chemistry Letters (1995) 453-454, Doi et al. J. of Chem. Engg of Japan 36, 5, (2003), 548 556], Fe-Ferrierite catalysts [I. Melia' n-Cabrera et al. Catal. Commun. 6 (2005) 301-305]. In all of these experiments catalytic decomposition was carried out at temperatures above 700° C. which thus would require heating of the gas and the catalyst for effective catalytic decomposition. The presently described process on the other hand does not require such high temperature heating of the feed gas hence economic than the methods described in the published journals.
Reference also may be made to the patent of Hamon and Duclos (Fr patent no FR 2,847,830) who have used solid solution of mix oxides of cerium and zirconium in high temperature ranging from 700 to 1000° C. to decompose N2O to N2 and O2. The problem with zirconium oxide type catalysts is that they work only in high temperature and undergoes structural modification after some use hence not durable. Apart from structural degradation the high temperature associated with the system leads to sintering of oxide grains leading to porosity loss and effective specific surface. In the said patent the inventors have used crushed pellets of ZrO2 and CeO2 solid solution of particle size between 0.5 to 1 mm. The ZrO2:CeO2 ratio in the system was in the range somewhere between 80:20 and 20:80, in addition to it the catalyst contained other element like Yttrium in minor percentage composition. The catalyst thus contained toxic oxides like cerium oxide which almost invariably contains traces of thorium emitting alpha particles. The presently invented catalyst is free from such radioactive or heavy metal ion hazards. Similarly, reference also may be made to another U.S. Pat. No. 5,472,677 dated December, 1995 by Farris et al.) where it has been described invention of a N2O decomposition catalyst derived from Co—Al type LDH powders.
In all of these papers or patents the catalytic decomposition experiments were carried out by using either catalyst powders or pellets. Both of these forms of catalysts had their own distinct disadvantages e.g. powder form of catalysts give rise to high pressure drop and pellets on the other hand give rise to core sintering due to temperature gradient between the surface and the center. The coated honeycomb monolithic catalyst used in the present invention does not possess such disadvantages. Indeed the pressure drop across the small unitary parallel passages of honeycomb type ceramic monoliths is two to three times smaller than the same in a collection of spherical catalyst pellets of equivalent area.
Reference may be made about the use of coated honey-comb type monolithic catalyst containing LDHs as one of the coating component for the following U.S. Pat. No. 6,923,945 by Chen S F dated January, 2004 and No. 6,419,890 by Li Y dated July, 2002). In the first patent LDH has been added with other inorganic components alumina, zirconia, rare earth oxides and platinum metals etc. and applied as a coat to trap SOx so that actual ‘Three Way Catalyst’ is not destroyed. In the second patent also LDHs have been added in the form of wash coatings over ceramic honeycombs to alleviate decrease of activity of ‘Three Way Catalyst’, but in none of these catalysts novel particle-particle interaction like the one between negatively charged smectites with positively charged LDHs, as claimed in the present patent, to form stable gels have been used. It is to be noted here that in none of these catalysts LDH particles were disaggregated to their individual nano sheet levels by taking course to any physical or chemical route nor they were dispersed to a non flocculated, viscous state to coat as thin films over any favourably structured solid surface.
As the present patent is based upon this remarkable gelation property of LDH and swelling clay composites which pave the way for various positive advantages viz. the swelling clay component acting as a film producing ingredient upon which nano dimensional active mixed metal oxides of suitable bi and trivalent combinations having layered morphology are supported out in the form of billboards in the pathway of gas molecule streams, or the extremely good non-Newtonian flow behaviour helping in preparing reproducible green coats of desired thickness over suitable ceramic matrix by film forming techniques like dip or spin coating etc (FIG. 1).
As such LDH-clay composites have been reported by Goswamee et al. (Indian patent No 235052). The present process is a novel approach of mixed metal oxidic nano sheets coated monolithic catalysts useful for the decomposition of toxic N2O and a process for the preparation thereof, which have advantages like increased activity due to nano dimensionality, supported projection, layered morphology and tunable metal ion composition.